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MAYANK JAIN1, NIGEL H. WOODCOCK2 & SAMPAT K. TANDON1
Abstract: A Quaternary fluvial sequence along the M ahi River, Gujarat, western India, shows evidence for a discrete episode of normal faulting and related surface slumping. Four mapped NW-striking faults have throws ranging from I m to over 3 m, and have related growth folds in synchronous sediment units. A slump sheet moved tens of metres northeastward down the 0.7deg' dip slope of one tilt block during displacement on its bounding fault. The exposed part of the internally folded sheet is probably one side of a lobate slump mass, several hundred metres in width.
Fault activity began after deposition of a lower gravel, which regional correlations suggest is about 300 ka old. Faulting ended just before deposition of an upper gravel-sand complex dated by luminescence techniques in correlated sections at no younger than about 60 ka. The Mahi River section shows that neotectonic activity in Peninsular India, exemplified by the 1993 Killari (Latur) earthquake, has a mid- to late Pleistocene component.
Keywords: Gujarat, India, Quaternary, neotectonics, faults, slumping.
Surface fault breaks associated with the earthquakes at Killari (Latur 1993) and Allah Band (Kutch 1819), together with the instrumental record of the Jabalpur (1997) and other smaller shocks, provide evidence of continuing seismic faulting in central and western India, well to the south of the Himalayan seismic zone (Fig. I; Richter 1958; Chung 1993; Gupta, 1994; Chung et al. 1995; Rajendran et al. 1996; Seeber et al. 1996; Gupta et al. 1997; Rajendran & Rajendran 1998). This peninsular seismicity is broadly ascribed to N S compression between the continental collision zone to the north and the spreading ridge to the south (e.g. Khattri 1994; Gowd et al. 1996; Biswas & Majumdar 1997). The sites of Quaternary fluvial sedimentation in western India have been attributed to control by reactivation of basement faults seen as lineaments on satellite images (Pant & Chamyal 1990; Sareen et al. 1993) and Quaternary tilting has been inferred from changing drainage patterns in south India (Subrahmanya 1996). Despite this, little direct evidence of fault displacement has been detailed from Quaternary sequences in this region. This paper reports such evidence.
The Gujarat alluvial plains form the southern semi-arid desert margin to the Thar...